The present invention relates generally to position determination, and particularly to position determination using DTV signals.
There have long been methods of two-dimensional latitude/longitude position location systems using radio signals. In wide usage have been terrestrial systems such as Loran C and Omega, and a satellite-based system known as Transit. Another satellite-based system enjoying increased popularity is the Global Positioning System (GPS).
Initially devised in 1974, GPS is widely used for position location, navigation, survey, and time transfer. The GPS system is based on a constellation of 24 on-orbit satellites in sub-synchronous 12 hour orbits. Each satellite carries a precision clock and transmits a pseudo-noise signal, which can be precisely tracked to determine pseudo-range. By tracking 4 or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details are provided in B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C. 1996.
GPS has revolutionized the technology of navigation and position location. However in some situations, GPS is less effective. Because the GPS signals are transmitted at relatively low power levels (less than 100 watts) and over great distances, the received signal strength is relatively weak (on the order of xe2x88x92160 dBw as received by an omni-directional antenna). Thus the signal is marginally useful or not useful at all in the presence of blockage or inside a building.
There has even been a proposed system using conventional analog National Television System Committee (NTSC) television signals to determine position. This proposal is found in a U.S. Patent entitled xe2x80x9cLocation Determination System And Method Using Television Broadcast Signals,xe2x80x9d U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the analog TV signal described contains horizontal and vertical synchronization pulses intended only for relatively crude synchronization of the TV set sweep circuitry, and not suitable for precise positioning. Further, in 2006 the Federal Communication Commission (FCC) will consider turning off NTSC transmitters and reassigning that valuable spectrum so that it can be auctioned for other purposes deemed more valuable.
The above disclosures describe the use of the new American Television Standards Committee (ATSC) digital television (DTV) signals for position location. When these techniques are used to determine the position of a handheld unit such as a cellular telephone, the efficient use of the limited power supply of the handheld unit is important.
In general, in one aspect, the invention features a computer program product, apparatus, and method for use in determining the position of a user terminal. It includes receiving at the user terminal a digital television (DTV) broadcast signal transmitted by a DTV transmitter; tracking a periodic component of the DTV signal using a delay-lock loop (DLL), including selecting an observation interval based on the timing of the periodic component, and turning on a portion of the DLL during the observation interval, and turning the portion off otherwise; and determining a pseudo-range between the user terminal and the DTV transmitter based on the DTV broadcast signal; and wherein the position of the user terminal is determined based on the pseudo-range and a location of the DTV transmitter.
Particular implementations can include one or more of the following features. Implementations include determining the position of the user terminal based on the pseudo-range and the location of the DTV transmitter. Determining a position of the user terminal includes adjusting the pseudo-range based on a difference between a transmitter clock at the DTV transmitter and a known time reference; and determining the position of the user terminal based on the adjusted pseudo-range and the location of the DTV transmitter. The DTV broadcast signal is an American Television Standards Committee (ATSC) DTV signal, and the pseudo-range is determined based on a known digital sequence in the ATSC frame. The known digital sequence is a synchronization code. The synchronization code is a Field Synchronization Segment within an ATSC data frame. The synchronization code is a Synchronization Segment within a Data Segment within an ATSC data frame.
Determining a position of the user terminal includes determining an offset between a local time reference in the user terminal and a master time reference; and determining the position of the user terminal based on the pseudo-range, the location of the DTV transmitter, and the offset. Determining a pseudo-range includes correlating the DTV signal with a signal generated by the user terminal as the DTV signal is received to produce the pseudo-range.
Implementations include tracking the pilot signal of the DTV signal using a phase-lock loop; and wherein tracking the component of the DTV signal is based on the tracking of the pilot signal. Implementations include transmitting the pseudo-range to a location server configured to determine a position of the user terminal based on the pseudo-range and a location of the DTV transmitter. The position of the user terminal is determined by adjusting the pseudorange based on a difference between a transmitter clock at the transmitter of the broadcast analog television signal and a known time reference, and determining the position of the user terminal based on the adjusted pseudorange and the location of the TV transmitter. Implementations include determining a further pseudorange based on a further broadcast analog television signal; and projecting the pseudorange and the further pseudorange to an instant of time, thereby eliminating any first order term in the clock of the user terminal.
Advantages that can be seen in implementations of the invention include one or more of the following. Implementations of the invention may be used to position cellular telephones, wireless PDA""s (personal digital assistant), pagers, cars, OCDMA (orthogonal code-division multiple access) transmitters and a host of other devices. Implementations of the inventions make use of a DTV signal which has excellent coverage over the United States, and the existence of which is mandated by the Federal Communication Commission. Implementations of the present invention require no changes to the Digital Broadcast Stations.
The DTV signal has a power advantage over GPS of more than 40 dB, and substantially superior geometry to that which a satellite system could provide, thereby permitting position location even in the presence of blockage and indoors. The DTV signal has roughly six times the bandwidth of GPS, thereby minimizing the effects of multipath. Due to the high power and low duty factor of the DTV signal used for ranging, the processing requirements are minimal. Implementations of the present invention accommodate far cheaper, lower-speed, and lower-power devices than a GPS technique would require.
In contrast to satellite systems such as GPS, the range between the DTV transmitters and the user terminals changes very slowly. Therefore the DTV signal is not significantly affected by Doppler effects. This permits the signal to be integrated for a long period of time, resulting in very efficient signal acquisition.
The frequency of the DTV signal is substantially lower that that of conventional cellular telephone systems, and so has better propagation characteristics. For example, the DTV signal experiences greater diffraction than cellular signals, and so is less affected by hills and has a larger horizon. Also, the signal has better propagations characteristics through buildings and automobiles.
Unlike the terrestrial Angle-of-Arrival/Time-of-Arrival positioning systems for cellular telephones, implementations of the present invention require no change to the hardware of the cellular base station, and can achieve positioning accuracies on the order of 1 meter. When used to position cellular phones, the technique is independent of the air interface, whether GSM (global system mobile), AMPS (advanced mobile phone service), TDMA (time-division multiple access), CDMA, or the like. A wide range of UHF (ultra-high frequency) frequencies has been allocated to DTV transmitters. Consequently, there is redundancy built into the system that protects against deep fades on particular frequencies due to absorption, multipath and other attenuating effects.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.